Voltage supply circuit for vacuum tubes



Aug. 8, 1950 D. K. FROMAN 2,517,863

VOLTAGE SUPPLY CIRCUIT FOR VACUUM TUBES Filed 001;. 12, 1944 x Reciz' icd 2 ACPour-erd'zyyqly l m m i INVENTOR. F] Darol K Human Patented Aug. 8, 1950 UNITED STATE VOLTAGE SUPPLY CIRCUIT FOR VACUUM i TUBES Darol K. Fronian, Denver, (3610., 'assignor to, the UnitedStates of America as represented by, the United States Atomic Energy Commission Application October 12, 1944, Serial No. 558,423

This invention relates to vacuum tube circuits and more particularly to circuit arrangements for supplying operating potentials to various electrodes of vacuumtubes under conditions which demand that certain of these potentials be maintained constant and that no undesired reactance be introduced by the source supplying these potentials.

In the electronicart circuit requirements often place a considerable burden on the designer, who in the attempt to'meet certain specifications, is faced with the difficulty of placing component elements possessing inherently disturbing electrical constants in a judicious manner. These electrical constants, are per se unalterable, such as the interelectrode capacities of vacuum tubes or the inductive or'capacitive reactance that the physical configuration of the components must necessarily introducej n A particular feature of this invention isithat the conditions above-referred to are alleviated with respect to specific'circuits wherein heretofore the source and the application of certain operating voltages has been a difiicult problem.

, The invention is particularly adaptable to vacuum tube circuits in which one or more multigrid tubes, such as a pentode or a tetrode is used in s'uchgmanner that the zero signal potential reference point is at the anode electrode. In these circuits it is difiicult to provide the operating potentials for associated control electrodes without disturbing the cathode impedance or interfering with coupling circuits-designedto be energized therefrom.

Another important feature of this invention is that such disturbing effects are eliminated by providing a circuit arrangement in which the necessary operating potentials are supplied isolated from the cathode circuit with respect to signal voltages or currents.

An advantage of this invention is that the expedients chosen toobtain the features referred to are simple, and at the same time, exhibit other ancillary beneficial features. Among these, in particular a significant advantage is that in operation the voltage supply is automatically held substantially constant within wide limits as to potential variations of another electrode taken'as a reference point. u v U a Essentially, the invention provides a novel cir Claims. (01.179-171) cuit arrangement for an electron discharge device having a plurality of control electrodes besides the principal anode and cathode. At least one of these controlelectrodes requires a substantially constant'potential With reference to one of the principal electrodes. The circuit includes a source of operating potentials a portion of which is applied to the control electrode. A current controlling element in this circuit is in turn controlled by variations'of potential of the principal electrode. v,In this manner the voltage applied to the control electrode is determined by the current controlling element and is maintained substantially constant.

Other features and advantages will be apparent from the following description of the'invention, defined in particular by the appended claims and taken in connection with the accompanying drawings in which Figure 1- shows a vacuum tube circuit utilizing a single pentode with the output impedancein the cathode circuit wherein, in accordance withthe invention, the screen voltage is maintained substantially constant and the source doesnot interfere'with the cathode impedance. 5

Figure 2 shows a modificationof the invention applied to a vacuum tube amplifier wherein the anodelcoupling'impedance of the-amplifying tube comprises the dynamic anode resistance of a second vacuum tube.

shunt capacity of the heater circuit substantially reduced. v f

Referring to the figures the electrical system shown in Figure 1 comprises the vacuum tube l connected as a resistance coupled amplifying stage in which theinput circuit between grid electrode 2 and cathode '3 includes the resistor 4 The signal input is applied ventional manner to the cathode 3.- The output circuit is formed by the resistors 5 and 6 to which the output terminals I0 and Il connect. The anode potential source is shown here by the bat,-

tery 12. The latter repres ents by way of example, a source of direct current potential and may be replaced by any. suitable type of direct current supply. Th positive terminal ofgthe battery 12 The screen voltage for the resistance tube is held constant and the effective limitations. The by-pass condenser would be effectively in shunt With the output circuit and if the operation is in the high frequency ranges, the

resultant change in output im-pedancewould limit the usable range of amplification. In view of the fact that the potential of the screen grid electrode I6 is to be held constant and the-cathode electrode is the reference point a battery .as a source for this potential would have to be connected between the cathode 3 andthescreengrid electrode 16. No matter how small this. battery is in practice the physical size, despite its most judicious placement, introduces sufficient capacity to'ground to limit the frequency range of operation. Electrical power supplies would necessarily introduce even a larger unwanted capacity.

-A-supply for the screen grid'electrode I6 is obtained, in accordance with the invention, from a voltage divider in which one element of the dividing network is the anode-cathode current path "of a vacuum tube 18. The other element is a high-resistor 19. 'As will be seen, essentially the vacuum tube 18 performs the function of a cathode follower circuit. The cathode 20 connects to the negative terminal of thebattery [2 through the cathode resistor l9 whereas the anode 2| is connected to the positive terminal. The screen grid 18 is directly connected to the cathode-20. The cathode-follower is driven in accordance with potential variations of the cathode '3' of the vacuum tube I. In view of the fact that the grid electrode 23 of the tube I8 connects to the cathode 3, a source of potential of approximately the value-of the requiredscreen grid potential is in series between the grid 23 of the tube I8 and the cathode 3 of the tube'l. This source is shown by the battery 25. It is to be noted that there is no current fiow in the grid circuit of the tube l8 consequently, the battery 25 delivers only voltage and therefore can be made physically extremely small. It can also be replaced veryadvantageously by small primary cells as-will be described later.

In the operation ofthe circuit, assuming that no input signal is applied and static conditions prevail, the current flow through the cathodefollower 'tube l8 as determined by the applied grid bias voltage is of such magnitude that the IRdrop'of the resistor It applies to the screen grid! E5 the-required potential difference. When a signal is applied between input terminals 1 and 8 whether it is a uni-directional voltage or an alternating current, the change of potential of the'cathode 3 controls the actionof the vacuum tube Hi. This in turn will change the current through the resistor I9 so that the effective potential of the screen grid electrode is is also alteredin accordance with current fluctuations in-the cathode circuit 'of the vacuum tube In other words, as the voltage across the resistors 5 and -6 increases due to an increase of anode current of the tube l,'the bias on thegrid 23 becomes also more positive and a corresponding increase in current will increase the voltage drop across the resistor l9. Alternately, considering a reverse operation, when the voltage across the cathode resistors 5 and 6 decreases a similar decrease of current will be initiated in the voltage divider network; that is, in the cathode-follower the applied screen grid potential will then be reduced. With respect to the cathode 3 as a reference point, the regulating action is compensative in that the effective screen grid potential will be held substantially constant. It is to be notedthat the potential of the screen grid l6 of tube l follows the fluctuations of the cathode 3 very rapidly, for the cathode-follower tube I8 acts as a generator with extremely low internal impedance. This low impedance also insures that the potential difierence between the screen grid I6 and the cathode 3 is very nearly constant for wide fluctuations of screen grid current.

'Referringto Figure 2, the invention is applied to a single stage high gain amplifier of the type wherein the output impedance is formed by the dynamic resistance of a vacuum tube. In other words, a vacuum tube replaces the conventional load resistance in the anode circuit of the amplifyin'g tube. Amplifiers'of this type are known in the art for example, the one disclosed in U. S. Patent 1,548,952 to Mills. It was found that very high gain per stage can be obtained approaching 'the amplification factor of the tube. The reason for this is that the second tube taking place of the anode resistance offers a comparatively low resistance for the static anode current and amuch higher effective resistance for the signal'component namely, thedynamic resistance of the tube.

The second vacuum tube which will be referred to as .the"resistance tube by virtue of its connection has its cathode at a high potential with respect to signal voltages. Consequently, operating potentials for other electrodes, for example, the screen grid electrode must be applied with reference "to thecathode which, as stated before,

, changes in accordance with signal potentials.

Separate batteries used heretofore to supply this voltage. introduced acertain inherent capacity to ground which has a deleterious effect. The useful amplification, particularly in the higher frequency ranges, was therefore seriously limited.

:The amplifying tube 30 is connected in the circuit in a conventional manner with respect to the various electrodes except the anode 3!. The cathode32 returns to the common or ground potential point of the system through a cathode resistor 33 which is by-passed by a condenser 34. The suppressor electrode'35 returns to the oathode 32. The input circuit of the amplifier between terminals 31 and 38 includes the grid resistorv 39 connected to the grid 40 and ground. The output circuit is obtained between terminals 59and 60 connected to the anode 3| and ground, respectively.

A portion of a conventional rectified, filtered alternating current power supply is shown for the operating voltage source. The rectifier portion is omitted for the sake of simplicity and only the filter choke M is shown in the positive side of the supply, by-passed .by filter condensers 15 andflifi. A voltage divider comprising resistors 4'11 and 48 in series between output terminals of the supply provides the required screen grid potential for the tube '30. The junction point of the'resistors-fl a'nd'tB connects to the screen grid electrode 36. This connection is by-passed by tice;

The amplifier shown in .Fig'iire2 when energized from an alternating current supply-would havelthe heater elements of the tubes connected tosuitably proportioned transformer =windings.

I I invention is not concerne'd with this portion of the circuit which follows conventional practherefore, the heater circuit of the tubes as well as their power supplyare omitted. However, the heater and the reason for this will be better understood when following the description of the invention. The resistance tube ll,has its anode 5|,connected directly to the high potential side of the power supply. The suppressor grid 52 returns to the cathode 53 which connects to the anode 3| of the tube 30 through a resistor 54. The latter provides the required bias potential for the grid 55 which is connected to the terminal of the resistor 54 which is connected to anode 3| of tube 30.

The operating potential for the screen grid 51 is obtained in the same manner as shown in connection with the description of Figure 1. Identical parts are marked with similar reference characters bearing primary indices. The vacuum tube I8 is connected as a cathode-follower across the terminals of the anode voltage supply. The resistor 59' between cathode 20' returns to the negative terminal whereas the anode 2i to the positive terminal of the supply.

The grid 23' of the tube l8 connects to the cathode 53 through a bias supply source 25 shown here by a number of primary cells connected in series. These cells are used in the art to supply potentials in circuits where practically no current is drawn. For this reason they are known as bias cells. Their small size and long life makes them particularly adaptable where a uniform and steady voltage source is required with a minimum of bulk so as not to introduce undesired capacities.

The screen grid 5'! connects directly to the cathode 2| and to the heater circuit indicated by a center tapped resistor connected across the conductors leading to the filament 49 of the tube 59. The arrows terminating the heater circuit wires indicate that this is to be connected to a filament transformer, not shown here.

Referring to the operation of the circuit shown in Figure 2, the vacuum tube 30 connected as an amplifier has its anode potential supplied, as previously stated, through the anode to cathode space current path of the vacuum tube 50. The latter, by virtue of the controlled screen grid potential will maintain a uniform load impedance. The voltage between cathode 53 and screen grid 5'! is held substantially constant, in view of the fact that potential variations at signal voltage rate of the anode 3i will be applied to the grid 23' of the tube l8. These variations control the current in the cathode resistance 19 so that the voltage drop thereacross supplies to the screen grid 51 a control potential tending to maintain the effective voltage between the cathode 53 and the screen grid 51 constant. It is to be noted that the heater 49 being connected to the screen grid electrode 51 will also be held at a uniform potential. This connection eliminates the by-passing elfect which would be produced if the heater 49 were held at ground potential. The effective frequency range of the amplifier is thereby considerably increased.

The cathode-follower voltage control presents an extremely high impedance in the circuit of the control grid 23 connected to the cathode 53; therefore, although this connection is at a high "circuit ofbne tube is indicated potential level withrespectto signal voltages, the

ing saidlast mentioned'tube with operating ..po-

t'entials including screen grid potential of substantially constantvalue with respect to the oathode of said second tube, said meansincluding a source of potential, a vacuum tube connected as a cathode-follower between terminals of said source, circuit means for applying the potential drop across the coupling element of said cathodefollower to the screen grid electrode of said second tube and circuit means for controlling said cathode-follower in accordance with changes of potential of the cathode of said second tube.

2. In an amplifying system wherein the anode coupling impedance of an amplifying vacuum tube comprises the dynamic anode resistance of a second vacuum tube, means for supplying said last mentioned tube with operating potentials including screen grid potential of substantially constant value with respect to the cathode of said second tube, said means including a source of direct current potential, a vacuum tube having anode, cathode, and grid electrodes connected as a cathode-follower in series with a resistance between terminals of said source, circuit means for applying the potential drop of said cathode-follower to the screen grid of said second tube comprising a connection between the cathode of said cathode-follower and said screen grid electrode and circuit means for driving said cathode-follower in accordance with changes of potential of the cathode of said second tube comprising a connection between said last mentioned cathode and the grid electrode of said cathode-follower.

3. In an amplifying system wherein the anode coupling impedance of an amplifying vacuum tube comprises the dynamic anode resistance of a second vacuum tube, means for supplying said last mentioned tube with operating potentials including screen grid potential of substantially constant value with respect to the cathode of said second tube, said means including a source of direct current potential, a vacuum tube having anode, cathode, and grid electrodes connected as a cathodefollower in series with a resistance between terminals of said source, circuit means for applying the potential drop of said cathode-follower to the screen grid of said second tube comprisin a connection between the cathode of said cathodefollower and said screen grid electrode and circuit means for driving said cathode-follower in accordance with changes of potential of the cathode of said second tube comprising a connection including a source of bias potential between said last mentioned cathode and the grid electrode of said cathode-follower.

4. In an amplifying system wherein the anode coupling impedance of an amplifying vacuum tube comprises the dynamic anode resistance of a second vacuum tube, whereby the cathode of said last mentioned tube is at signal potential, a heater for said cathode, a circuit for supplying screen grid potential for said second tube and means for reducing the deleterious effect of the inherent capacity of said heater including a circuit for applying said screen grid potential to said heater.

5. In an amplifying system wherein the anode coupling impedance of an amplifying vacuum tube 7 comprises the 1 dynamic ano de resistance of a. second vacuum tube, whereby the cathode. iofasaid last mentioned tube is at signal potential, aheater for said cathode, a. circuit 'for supplying screen -gri'd potential 'for' said second tube and means for reducing the deleterious effect of the inherent capa'oityof said heater including a circuit interconnecting said heater and the screen grid electrode'of said tube.

- DAROL K; FROMAN.

REFERENCES CITED The following references are of record in the file of this patent:

" Number Numb er FUNITED STATES PATENTS Name Date Mills Aug. 11, 1925 Tellegen May 15, 1934 Brailsford Dec. 13, 1938 Eaton Dec.22, 1942 Bowman Aug. 10, 1943 Levy Feb. 25, 1947 FOREIGN PATENTS 1 Country Date 7 Germany June 5,1936 

